January 13 1 Crystal Field Theory

23.4 Crystal Field Theory

The relationship between Colors, Metal Complexes and Gemstones

Dr. Fred O. Garces Chemistry 201 Miramar College

400! 500! 600! 800!

January 13 2 Crystal Field Theory

Transition Metal Gems

Gemstone owe their color from trace transition-metal ions Corundum mineral, Al2O3: Colorless

Cr g Al : Ruby Mn g Al: Amethyst Fe g Al: Topaz Ti &Co g Al: Sapphire Beryl mineral, Be3 Al 2Si6O18: Colorless Cr g Al : Emerald Fe g Al : Aquamarine

January 13 3 Crystal Field Theory

Crystal-Field Theory Coordination Chemistry and Crystal Field Theory

January 13 4 Crystal Field Theory

Crystal-Field Theory Model explaining bonding for transition metal complexes

• Originally developed to explain properties for crystalline material • Basic idea:

Electrostatic interaction between lone-pair electrons result in coordination.

January 13 5 Crystal Field Theory

Energetics CFT - Electrostatic between metal ion

and donor atom

i) Separate metal and ligand high energy

ii) Coordinated Metal - ligand stabilized

iii) Destabilization due to ligand to metal’s d-electron repulsion

iv) Splitting due to octahedral field.

i

ii

iii

iv

January 13 6 Crystal Field Theory

Ligand-Metal Interaction CFT - Describes bonding in Metal Complexes Basic Assumption in CFT: Electrostatic interaction between ligand and metal

d-orbitals align along the octahedral axis will be affected the most.

More directly the ligand attacks the metal orbital, the higher the the energy of the d-orbital.

In an octahedral field the degeneracy of the five d-orbitals is lifted

January 13 7 Crystal Field Theory

d-Orbitals and Ligand Interaction (Octahedral Field)

Ligands approach metal

d-orbitals not pointing directly at axis are least affected (stabilized) by electrostatic interaction

d-orbitals pointing directly at axis are affected most by electrostatic interaction

January 13 8 Crystal Field Theory

Splitting of the d-Orbitals Octahedral field Splitting Pattern:

The energy gap is referred to as Δ (10 Dq) , the crystal field splitting energy.

The dz2 and dx2-y2 orbitals lie on the same axes as negative charges. Therefore, there is a large, unfavorable interaction between ligand (-) orbitals. These orbitals form the degenerate high energy pair of energy levels.

The dxy , dyx and dxz orbitals bisect the negative charges. Therefore, there is a smaller repulsion between ligand & metal for these orbitals. These orbitals form the degenerate low energy set of energy levels.

January 13 9 Crystal Field Theory

Magnitude of CF Splitting (Δ or 10Dq) Color of the complex (as well as the magnetism) depends on magnitude of Δ

1. Metal: Larger metal g larger Δ Higher Oxidation State g larger Δ 2. Ligand: Spectrochemical series Cl- < F- < H2O < NH3 < en < NO2

- < (N-bonded) < CN-

Weak field Ligand: Low electrostatic interaction: small CF splitting. High field Ligand: High electrostatic interaction: large CF splitting.

Spectrochemical series: Increasing Δ

January 13 10 Crystal Field Theory

Electron Configuration in Octahedral Field Electron configuration of metal ion:

s-electrons are lost first. Ti3+ is a d1, V3+ is d2 , and Cr3+ is d3

Hund's rule:

First three electrons are in separate d orbitals with their spins parallel.

Fourth e- has choice: Higher orbital if Δ is small; High spin Lower orbital if Δ is large: Low spin.

Weak field ligands

Small Δ , High spin complex Strong field Ligands

Large Δ , Low spin complex

January 13 11 Crystal Field Theory

High Spin Vs. Low Spin (d1 to d10) Electron Configuration for Octahedral complexes of metal ion having d1 to d10 configuration [M(H2O)6]+n. Only the d4 through d7 cases have both high-spin and low spin configuration.

Electron configurations for octahedral complexes of metal ions having from d1 to d10 configurations. Only the d4 through d7 cases have both high-spin and low-spin configurations.

January 13 12 Crystal Field Theory

Color Absorption of Co3+ Complexes The Colors of Some Complexes of the Co3+ Ion

The complex with fluoride ion, [CoF6]3+ , is high spin and has one absorption band. The other complexes are low spin and have two absorption bands. In all but one case, one of these absorptions is in the visible region of the spectrum. The wavelengths refer to the center of that absorption band.

Complex Ion Wavelength of Light absorbed (nm)

Color of Light absorbed

Color of Complex

[CoF6] -3 700 Red Green [Co(C2O4)3] 3+ 600, 420 Yellow, violet Dark green [Co(H2O)6] 3+ 600, 400 Yellow, violet Blue-green [Co(NH3)6] 3+ 475, 340 Blue, violet Yellow-orange

[Co(en)3] 3+ 470, 340 Blue, ultraviolet Yellow-orange

[Co(CN)6] 3+ 310 Ultraviolet Pale Yellow

January 13 13 Crystal Field Theory

Colors & How We Perceive it

800!

430!

650! 580!

560!

490!

Artist color wheel showing the colors which are complementary to one another and the wavelength range of each color.

400!

January 13 14 Crystal Field Theory

Black & White

If a sample absorbs all wavelength of visible light, none reaches our eyes from that sample. Consequently, it appears black.

When a sample absorbs light, what we see is the sum of the remaining colors that strikes our eyes.

If the sample absorbs no visible light, it is white or colorless.

January 13 15 Crystal Field Theory

Absorption and Reflection

If the sample absorbs all but orange, the sample appears orange.

Further, we also perceive orange color when visible light of all colors except blue strikes our eyes. In a complementary fashion, if the sample absorbed only orange, it would appear blue; blue and orange are said to be complementary colors.

750!

430!

650! 580!

560!

490!

400!

January 13 16 Crystal Field Theory

Light absorption Properties of Metal Complexes Recording the absorption Spectrum and analyzing the spectra.

The absorption of light at 500 (- 600nm) nm (green-yellow) , result in the observation of the complement color of 700nm (violet - red) according to the color wheel.

January 13 17 Crystal Field Theory

Complex Influence on Color Compounds of Transition metal complexes solution.

[Fe(H2O)6]3+

[Co(H2O)6]2+ [Ni(H2O)6]2+

[Cu(H2O)6]2+ [Zn(H2O)6]2+

800!

430!

650! 580!

560!

490!

400!

January 13 18 Crystal Field Theory

Color Absorption of Co3+ Complexes

The Colors of Some Complexes of the Co3+ Ion

The complex with fluoride ion, [CoF6]3+ , is high spin and has one absorption band. The other complexes are low spin and have two absorption bands. In all but one case, one of these absorption is in the visible region of the spectrum. The wavelengths refer to the center of that absorption band.

Complex Ion Wavelength of Light absorbed (nm)

Color of Light absorbed

Color of Complex

[CoF6] -3 700 Red Green [Co(C2O4)3] 3+ 600, 420 Yellow, violet Dark green [Co(H2O)6] 3+ 600, 400 Yellow, violet Blue-green [Co(NH3)6] 3+ 475, 340 Blue, violet Yellow-orange

[Co(en)3] 3+ 470, 340 Blue, ultraviolet Yellow-orange

[Co(CN)6] 3+ 310 Ultraviolet Pale Yellow

January 13 19 Crystal Field Theory

Octahedral, Tetrahedral & Square Planar

CF Splitting pattern for various molecular geometry

M

dz2 dx2-y2

dxz dxy dyz

M

dx2-y2 dz2

dxz dxy dyz M

dxz

dz2

dx2-y2

dxy

dyz

Octahedral Tetrahedral Square planar

Pairing energy Vs. Δ Weak field Δ < PE Strong field Δ > PE

Small Δ g High Spin Mostly d8

(Majority Low spin) Strong field ligands i.e., Pd2+, Pt2+, Ir+, Au3+

January 13 20 Crystal Field Theory

Summary

Crystal Field Theory provides a basis for explaining many features of transition-metal complexes. Examples include why transition metal complexes are highly colored, and why some are paramagnetic while others are diamagnetic. The spectrochemical series for ligands explains nicely the origin of color and magnetism for these compounds. There is evidence to suggest that the metal-ligand bond has covalent character which explains why these complexes are very stable. Molecular Orbital Theory can also be used to describe the bonding scheme in these complexes. A more in depth analysis is required however.